Intrinsic defects on α, γ and δ-CsPbI (001) surfaces and implications for the α/γ to δ phase transition.

Compared with traditional organic-inorganic hybrid perovskites, CsPbI is considered to be a better solar photovoltaic absorption material. However, under environmental conditions, it will undergo the phase transition from the α phase to the γ phase and finally to the non-perovskite phase (δ), especially in a humid environment. Considering the important role of surface intrinsic defects in the phase transition process, we investigated the intrinsic defects on α, γ and δ-CsPbI (001) surfaces by first-principles calculations based on density functional theory (DFT). The formation energy of most defects on the surface is similar to that in the bulk in all three phases except for V and V. The formation energy of V and V on the α-CsPbI (001) surface is significantly increased, and the formation energy of V on the γ-CsPbI (001) surface is also increased, due to the relaxation and distortion of the surface Cs and the Pb-I octahedron. The formation energy of interstitial defects on the α-CsPbI (001) surface is the lowest due to the remaining large dodecahedral void, even though the Pb-I octahedron distortion has largely enhanced the stability of the α-CsPbI (001) surface. The formation energy of V is the lowest in all three phases, indicating that Cs ions in CsPbI are indeed flexible in CsPbI. The results are expected to provide a theoretical basis and guidance for the stability improvement of all-inorganic halide perovskites especially in a humid environment.